1. Field of the Invention
The present invention relates to a method for producing a helical spring and an apparatus for producing the same, and more particularly to the method and apparatus for producing the helical spring, with at least a warm setting process applied to a coiled wire.
2. Description of the Related Arts
As for methods for producing helical springs, a method for producing the same by cold working and a method for producing the same by hot working are known heretofore. Various types of coiling machines are on the market for use as a machine for producing the helical springs by the cold working. In Japanese Patent Laid-open Publication Nos.6-106281, 6-294631, 7-248811 and 9-141371, for example, the coiling machines are disclosed, and processes for controlling them are proposed. The basic structure of those coiling machines is based upon bending and twisting an element wire while feeding the wire, to produce the helical springs, with a machine accuracy improved by means of numerical control (NC). On the other hand, in accordance with recent progress of analytic technology, it is now possible to perform various simulations with respect to a certain spring-shaped model, and to design products on the basis of the result of the analysis. For example, it is possible to define a shape of a spring having a certain spring property, through FEM analysis.
In the case where the helical springs are produced by the coiling machines, however, mainly employed is a so-called try and error method for producing a prototype of the helical spring temporarily and forming it in a certain shape, with the dimension of the prototype being checked. In this case, although the coiling machines are driven according to the numerical control (NC), the data are input into the machines in dependence upon intuition or knack of operators. Therefore, measurements are made partially, so that overall shape of the product can not be ensured, and eventually caused is such a problem that if its shape is complex, a duration for producing the prototype will be prolonged.
According to the machine disclosed in the Japanese Patent Laid-open Publication No.7-248811 as described above, it was proposed to identify a part of the data to be corrected and confirm the data easily, in view of a prior automatic programming machine for use in a helical spring forming machine. In that publication, it is stated that a shape of a helical spring produced by the prior machine was slightly different from a shape of an originally designed spring in general, so that it was necessary for an operator to identify a part of the shape to be corrected on the basis of the image obtained through the data shown on a display, whereby an error was likely caused. In order to solve the problem as described above, it is proposed that the shape of the spring is shown on the display, then markers indicative of the part of the data to be corrected, and integrated number of coils (turns, or wind) are displayed, and that the data are input by the operator, watching the shape of the spring.
Although, improvements have been made with respect to the control of the coiling machines, as described in the above publications, they are limited to the improvements from the view point of controlling the machines, so that they have not reached to a level of creating a working process for forming the objects to be worked into those of desired shapes, which can be done by an ordinary machinery working process. This is because the problem is resulted from specific issues on the helical spring as follows:
At the outset, when the helical spring is produced by the cold working, an elastic deformation is necessarily caused, to create a spring-back. Therefore, it is difficult to estimate a position of a working tool, and an appropriate distance to move the same, unlike a cutting process and so on. In addition, the amount of spring-back is varied in dependence upon hardness of the element wire, and the shape of the helical spring. Especially, the finished compression helical spring is likely to cause a contact between the neighboring coils, so that it was very difficult to ensure a desired spring property. In view of those matters, generally employed is a method for obtaining the NC data by measuring the size of the actually produced prototype.
Furthermore, the dimension of the spring provided when designed and the dimension of the spring formed by the coiling machine do not coincide with each other. For example, comparing with diameters of coils which are provided to indicate a desired shape on a three-dimensional coordinate when the spring is designed, the diameters which are provided when the spring is formed are to be made larger, by a distance moved in the axial direction according to a lead. In addition, the feeding amount of the element wire (material) and the number of coils when worked (positions to be worked) do not coincide with each other, to cause a phase difference between the feeding amount of the element wire and bending positions or twisting positions. The number of coils (or turns) as described above is used for identifying the position to be worked, from the coil end, for example. Also, after the spring was formed by the coiling machine, generally a temper process (i.e., low temperature heat-treatment, simply referred to as heat-treatment) is applied to the spring, so as to cancel working stress applied thereto. Therefore, it is necessary to estimate a change in shape of the spring, before working it.
From the foregoing reasons, it was impossible in the prior arts to accurately identify the actual position of the target to be formed, which should correspond to the position of the desired shape on the coordinates. Therefore, the prototype was made by workers in dependence upon their intuition and knack, so that the spring was produced by a repetition of the try and error. As a result, the coiling machine capable of performing the numerical control could not be operated to fully use its inherent function, so that its operation was not far beyond a range of manual operation. In view of these, one of the inventers of the present application proposed a method for producing a helical spring by cold working, with an element wire bent and twisted while the wire being fed, wherein a target helical spring of a desired shape set in advance can be produced automatically and accurately, in a patent application filed in Japan as JPA2000-319745, and its corresponding applications filed in the U.S.A. as Ser. No. 09/976,158, and filed with European Patent office as 01124867.
Recently, in addition to the temper process as described above, it has been required to perform a warm setting process (or, called as hot setting), which will cause a large change in shape of the helical spring. Therefore, in order to produce the helical spring with a proper shape and accurate dimensions, it is necessary to consider not only the change in shape during the coiling process, but also the change in shape during the whole process for producing the helical spring, including an after-treatment such as the warm setting process. The after-treatment includes the temper process as described above, warm setting process for improving an anti-fatigue property, shot peening process for improving fatigue strength, coating process for improving an anticorrosion property, and the like, so that a plurality processes have to be made after the coiling process. In other words, in order to ensure a certain shape of a finished helical spring, it is necessary to evaluate a possible effect to the shape caused by the after-treatment including the warm setting process. In the prior application as described above, a practical countermeasure enough for reducing the effect especially caused by the warm setting process has not been disclosed in detail. It is preferable to produce the helical spring, with a proper correction applied for minimizing an error to a fundamental data, in accordance with the after-treatment including the warm setting process.